This invention relates to optical receivers for detecting coherent light signals comprising phase modulated (key-shifted) carrier signals. It is well known for such optical receivers to include a local oscillator the output signal from which is mixed with an incoming modulated coherent light signal to enhance the receiver sensitivity.
In the case of homodyne optical receivers the output from the local oscillator has a frequency which is the same as that of the incoming coherent light signal whereas in the case of heterodyne optical receivers the frequency of the local oscillator differs from that of the coherent light signal by an amount greater than the maximum modulation rate.
Since homodyne receivers require the local oscillator and incoming coherent light signal frequencies to be identical, the phase of the local oscillator needs to be closely controlled with respect to the incoming coherent light signal which in turn necessitates a more complex and costly local oscillator providing phase-locking facilities. However, the homodyne receiver does have greater sensitivity (e.g. 3 dB more sensitive) than the heterodyne receiver due to side-band elimination and moreover, it does not require the detector speed of response to be as high as the heterodyne receiver.